Role of malectin in Glc(2)Man(9)GlcNAc(2)-dependent quality control of α1-antitrypsin.

Malectin was first discovered as a novel endoplasmic reticulum (ER)-resident lectin from Xenopus laevis that exhibits structural similarity to bacterial glycosylhydrolases. Like other intracellular lectins involved in glycoprotein quality control, malectin is highly con - served in animals. Here results from in vitro membrane-based binding assays and frontal af- finity chromatography confirm that human malectin binds specifically to Glc 2Man9GlcNAc2 (G2M9) N-glycan, with a Ka of 1.97 × 10 5 M −1 , whereas binding to Glc1Man9GlcNAc2 (G1M9), Glc3Man9GlcNAc2 (G3M9), and other N-glycans is barely detectable. Metabolic labeling and immunoprecipitation experiments demonstrate that before entering the calnexin cycle, the folding-defective human α1-antitrypsin variant null Hong Kong (AT NHK ) stably associates with malectin, whereas wild-type α1-antitrypsin (AT) or N-glycan-truncated variant of AT NHK (AT NHK -Q3) dose not. Moreover, malectin overexpression dramatically inhibits the secretion of AT NHK through a mechanism that involves enhanced ER-associated protein degradation; by comparison, the secretion of AT and AT NHK -Q3 is only slightly affected by malectin overex- pression. ER-stress induced by tunicamycin results in significantly elevated mRNA transcrip- tion of malectin. These observations suggest a possible role of malectin in regulating newly synthesized glycoproteins via G2M9 recognition.